encrypt-xml.cs

public static void Encrypt(XmlDocument Doc, string ElementName, SymmetricAlgorithm Key)
{
    if (Doc == null)
        throw new ArgumentNullException("Doc");
    if (ElementName == null)
        throw new ArgumentNullException("ElementToEncrypt");
    if (Key == null)
        throw new ArgumentNullException("Alg");

    XmlElement elementToEncrypt = Doc.GetElementsByTagName(ElementName)[0] as XmlElement;
    if (elementToEncrypt == null)
    {
        throw new XmlException("The XML element was not found");
    }

    EncryptedXml eXml = new EncryptedXml();

    byte[] encryptedElement = eXml.EncryptData(elementToEncrypt, Key, false);

    EncryptedData edElement = new EncryptedData();
    edElement.Type = EncryptedXml.XmlEncElementUrl;

    string encryptionMethod = null;

    if (Key is Aes)
    {
        encryptionMethod = EncryptedXml.XmlEncAES256Url;
    }
    else
    {
        throw new CryptographicException("The specified algorithm is not supported.");
    }

    edElement.EncryptionMethod = new EncryptionMethod(encryptionMethod);
    edElement.CipherData.CipherValue = encryptedElement;
    EncryptedXml.ReplaceElement(elementToEncrypt, edElement, false);
}